FFmpeg and qt, Unable to find a suitable output format for '>'

Question

Hi all,

I'm trying to execute a ffmpeg operation through Qt

I would like to execute this line :

./ffmpeg -t 10 -i temp1 -f mpeg  -  >  temp2

When I execute through the terminal, it works perfectly fine.

How ever when I launch it through Qt like this :

 QProcess    *process = new QProcess();

 QString parameters("./ffmpeg -t 10 -i temp1 -f mpeg  - >  temp2");
 std::cout << process->execute(parameters) << std::endl;

I get an Unable to find a suitable output format for '>' any body has the idea of why ?

Answer 1

When you type your original command in the shell, ffmpeg outputs to stdout. The shell interprets the > redirection operator, so output is sent to temp2. The last thing ffmpeg sees is the -.

When you use QProcess like that, there is no shell, so ffmpeg sees > and temp2 as arguments, which it doesn't interpret as you expect. In this case, there's an easy solution. Just have ffmpeg output to temp2 instead of stdout:

QString parameters("./ffmpeg -t 10 -i temp1 -f mpeg temp2");
std::cout << QProcess::execute(parameters) << std::endl;

In other cases, you might have to execute a shell explicitly, something like:

QStringList paramList;
paramList << "-c" << "./ffmpeg -t 10 -i temp1 -f mpeg  - >  temp2";
std::cout << QProcess::execute("sh", paramList) << std::endl;

Note that QProcess::execute is static, so an instance is not needed (this applies no matter how you call execute).

Answer 2

When you run this from the shell, the shell interprets the '> temp 2' as a redirection and never passes that to the ffmpeg process. When you run process->execute() this function is passing everything after ./ffmpeg as command-line parameters. If you really want shell-redirection, you need to run a new shell and have that shell execute your command line. This might work:

QString parameters("/bin/sh -c '/ffmpeg -t 10 -i temp1 -f mpeg  - >  temp2'");
std::cout << process->execute(parameters) << std::endl;

I'm not familiar with the quoting rules for Qt, so this might not work perfectly, but basically you want to prepend /bin/sh -c and then quote the command you actually want to run including the stdout redirection.

Answer 3

Hi.

Here is another example:

QProcess p;
p.start("./ffmpeg", QStringList() << "-t 10" << "-i temp" << "-f mpeg");
p.waitForFinished();
qDebug() << p.readAllStandardOutput();

Where p.readAllStandardOutput() is a QByteArray and you can read with is all data available from the standard output of the process, for example write them to file:

QFile file("out.txt");
if (!file.open(QIODevice::WriteOnly | QIODevice::Text))
    return;

QTextStream out(&file);
out << p.readAllStandardOutput();

file.close();

Answer 4

Another remark: It's also a good idea to always use the QStringList variant to pass args instead of a plain string - otherwise you'll run into quoting issues pretty soon, especially on Windows where paths with spaces in it are common. The QStringList variants do the right thing (TM).